This invention relates to catheters and more particularly to dilatation catheters which have an inflatable balloon portion which will not be displaced axially when inflated in a blood vessel to increase the patency thereof.
It is a well known medical practice to use balloon catheters for enlarging the luminal diameter of a blood vessel, for example, at a point of stenosis such as is produced by an accumulation of plaque. In one procedure, known as percutaneous transluminal coronary angioplasty, the patent is viewed on an x-ray imaging screen while a flexible guide wire is first introduced through the skin into a coronary artery of a patient, and is so manipulated as to travel therein and penetrate the lumen of an occluded portion of the artery. A guide catheter is then fed along the guide wire to a point in the artery which is just proximal of the occlusion. Finally, the dilatation catheter is sent along the guide wire, within the guide catheter, and into the artery of the patient to position the balloon portion of the catheter in the occluded portion of the artery.
One such dilatation catheter has a flexible shaft which includes an inner tube, or cannula, which can pass freely along the guide wire and a flexible outer tube which surrounds the inner tube and has an inner diameter which is somewhat larger than the outer diameter of the inner tube. A flexible balloon portion at the distal end of the outer shaft is sealed to the distal end of the inner tube. The balloon portion is capable of expansion when fluid under pressure is directed into the space between the outer tube or the shaft and the inner tube whereas the outer tube of the shaft is relatively more rigid and is not capable of such expansion.
When the balloon portion of the catheter has been correctly positioned as seen on the x-ray imaging screen, a radiopaque, fluid contrast medium is introduced under pressure into the space between the inner and the outer tubes to expand the balloon portion which presses against the occluded matter on the inside of the artery. The expansion of the balloon must be carefully controlled to prevent possible over-expansion and over-stressing of the wall of the catheter which might cause it to rupture, while putting sufficient force on the blood vessel to accomplish the objectives of the procedure. When the desired enlargement of the occluded portion of the artery is completed, the pressure on the fluid inside the catheter is relieved, the balloon shrinks, and the catheter is then removed.
In one catheter of the above type, the proximal end of the catheter is fitted to a mount which receives the proximal ends of the inner tube and of the shaft tube and seals them in spaced-apart relationship, while providing a passageway for supplying fluid under pressure to the space therebetween. When the catheter is pressurized, the inner tube shifts its position to accommodate the decrease in the length of the balloon which occurs when the balloon expands. Upon release of pressure in the catheter, the inner tube is returned to its original distal position so that he movement of the inner tube aids in reducing the diameter of the balloon to approximately its original diameter, easing removal of the catheter from the blood vessel.
Some of the known catheters of this type exhibit axial shrinkage of the balloon portion during inflation. In some prior art catheters, non-uniform axial shrinkage of the balloon during inflation results in undesirable curving of the distal portion of the balloon.
Accordingly, there is a need for a balloon catheter in which the external surface of the balloon portion does not rotate or change dimensions longitudinally during inflation and which, at the same time, does not experience significant axial displacement during inflation. Concomitant with the foregoing is a need for dimensional stability of the inflated balloon so that there is very little further expansion and stretch after the balloon reaches the desired inflated dimensions. In this way, overexpansion of the balloon and consequent damage to the vessel wall is minimized if the specified pressure is exceeded by mistake. The balloon which meets the foregoing needs should also be capable of rapid deflation and of subsequent complete recovery of original dimensions so as to allow easy and prompt retrieval when the procedure has been completed.